Laser tracker that uses a fiber-optic coupler and an achromatic launch to align and collimate two wavelengths of light

ABSTRACT

A coordinate measurement device sends a first beam of light having first and second wavelengths to a target point. The device includes a fiber-optic coupler that combines the first and second wavelengths and launches them through an achromatic optical element to produce collimated and aligned light. The device also includes first and second motors, first and second angle measuring devices, a distance meter, and a processor that determines 3D coordinates of the target point based on the measured distance and two angles.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. patent applicationSer. No. 13/431,494, filed Mar. 27, 2012, which claims the benefit ofU.S. Provisional Patent Application No. 61/592,049 filed Jan. 30, 2012,and U.S. Provisional Application No. 61/475,703 filed Apr. 15, 2011, theentire contents of all of which are herein incorporated by reference.

BACKGROUND

The present disclosure relates to a coordinate measuring device. One setof coordinate measurement devices belongs to a class of instruments thatmeasure the three-dimensional (3D) coordinates of a point by sending alaser beam to the point. The laser beam may impinge directly on thepoint or on a retroreflector target in contact with the point. In eithercase, the instrument determines the coordinates of the point bymeasuring the distance and the two angles to the target. The distance ismeasured with a distance-measuring device such as an absolute distancemeter or an interferometer. The angles are measured with anangle-measuring device such as an angular encoder. A gimbaledbeam-steering mechanism within the instrument directs the laser beam tothe point of interest.

The laser tracker is a particular type of coordinate-measuring devicethat tracks the retroreflector target with one or more laser beams itemits. Coordinate-measuring devices closely related to the laser trackerare the laser scanner and the total station. The laser scanner steps oneor more laser beams to points on a surface. It picks up light scatteredfrom the surface and from this light determines the distance and twoangles to each point. The total station, which is most often used insurveying applications, may be used to measure the coordinates ofdiffusely scattering or retroreflective targets. Hereinafter, the termlaser tracker is used in a broad sense to include laser scanners andtotal stations.

Ordinarily the laser tracker sends a laser beam to a retroreflectortarget. A common type of retroreflector target is the sphericallymounted retroreflector (SMR), which comprises a cube-cornerretroreflector embedded within a metal sphere. The cube-cornerretroreflector comprises three mutually perpendicular mirrors. Thevertex, which is the common point of intersection of the three mirrors,is located at the center of the sphere. Because of this placement of thecube corner within the sphere, the perpendicular distance from thevertex to any surface on which the SMR rests remains constant, even asthe SMR is rotated. Consequently, the laser tracker can measure the 3Dcoordinates of a surface by following the position of an SMR as it ismoved over the surface. Stating this another way, the laser trackerneeds to measure only three degrees of freedom (one radial distance andtwo angles) to fully characterize the 3D coordinates of a surface.

One type of laser tracker contains only an interferometer (IFM) withoutan absolute distance meter (ADM). If an object blocks the path of thelaser beam from one of these trackers, the IFM loses its distancereference. The operator must then track the retroreflector to a knownlocation to reset to a reference distance before continuing themeasurement. A way around this limitation is to put an ADM in thetracker. The ADM can measure distance in a point-and-shoot manner, asdescribed in more detail below. Some laser trackers contain only an ADMwithout an interferometer. U.S. Pat. No. 7,352,446 ('446) to Bridges etal., the contents of which are herein incorporated by reference,describes a laser tracker having only an ADM (and no IFM) that is ableto accurately scan a moving target. Prior to the '446 patent, absolutedistance meters were too slow to accurately find the position of amoving target.

A gimbal mechanism within the laser tracker may be used to direct alaser beam from the tracker to the SMR. Part of the light retroreflectedby the SMR enters the laser tracker and passes onto a position detector.A control system within the laser tracker can use the position of thelight on the position detector to adjust the rotation angles of themechanical axes of the laser tracker to keep the laser beam centered onthe SMR. In this way, the tracker is able to follow (track) an SMR thatis moved over the surface of an object of interest.

Angle measuring devices such as angular encoders are attached to themechanical axes of the tracker. The one distance measurement and twoangle measurements performed by the laser tracker are sufficient tocompletely specify the three-dimensional location of the SMR.

Several laser trackers are available or have been proposed for measuringsix, rather than the ordinary three, degrees of freedom. Exemplary sixdegree-of-freedom (six-DOF) systems are described by U.S. Pat. No.7,800,758 ('758) to Bridges et al., the contents of which are hereinincorporated by reference, and U.S. Published Patent Application No.2010/0128259 to Bridges et al., the contents of which are hereinincorporated by reference.

In the past, laser trackers that have absolute distance meters have usedmore than one wavelength. A visible light beam has been used for atleast two purposes—(1) providing a beam that lands on a positiondetector to enable tracking of a retroreflector target, and (2)providing a pointer beam by which a user may determine the pointingdirection of the tracker laser beam. An infrared light beam has beenused for an absolute distance meter. Such laser beams vary in wavelengthfrom 780 nm to 1550 nm. Difficulties that arise from the use of twodifferent wavelengths include (1) difficulty in obtaining precisealignment of the two different laser beams in traveling from the trackerto the retroreflector, (2) added expense resulting from the need to havetwo laser sources, extra beam splitters, and other components, and (3)larger required size of the laser beam because of the more rapidspreading of infrared wavelength beam of light compared to that of avisible beam of light. Because of the requirement to align the twodifferent beams of light, additional production steps have beenrequired, thereby increasing production costs. Furthermore, performanceof the tracker has never been quite as good as it could have been if thealignment were perfect. The larger required beam size has also meantthat beams were clipped by retroreflector targets, thereby resulting indecreasing accuracy in some cases and losing beams in other cases.

Another requirement of beams of light having multiple wavelengths isthat optics may not properly collimate each of the wavelengths. This isparticularly true if the optical configuration prevents each of thewavelengths from having its own optics to optimize collimation of thelight launched out of the tracker. What is needed is a laser trackercapable of launching multiple beams of light being mutually aligned andcollimated at a minimum of expense and complexity.

SUMMARY

According to an embodiment of the present invention, a coordinatemeasurement device is configured to send a first beam of light to atarget point, the target point having a position in space, the targetpoint returning a portion of the first beam as a second beam, themeasurement device including: a first light source configured to emit afirst light having a first wavelength; a second light source configuredto emit a second light having a second wavelength, the second wavelengthdifferent than the first wavelength; a fiber-optic coupler that includesat least a first port, a second port, and a third port, the first portconfigured to accept a first portion of the first light, the second portconfigured to accept a second portion of the second light, the thirdport configured to transmit a third light, the third light including aportion of the first portion and a portion of the second portion; anoptical system configured to transmit a portion of the third light outof the coordinate measurement device as the first beam, the opticalsystem including an achromatic optical element configured to collimatethe first beam at the first wavelength and the second wavelength; afirst motor and a second motor that together are configured to directthe first beam of light to a first direction, the first directiondetermined by a first angle of rotation about a first axis and a secondangle of rotation about a second axis, the first angle of rotationproduced by the first motor and the second angle of rotation produced bythe second motor; a first angle measuring device configured to measurethe first angle of rotation and a second angle measuring deviceconfigured to measure the second angle of rotation; a distance meterconfigured to measure a first distance from the coordinate measurementdevice to the target point based at least in part on a third portion ofthe second beam received by a first optical detector; and a processorconfigured to provide three-dimensional coordinates of the target point,the three-dimensional coordinates based at least in part on the firstdistance, the first angle of rotation, and the second angle of rotation.

According to another embodiment of the present invention, a method isprovided for measuring three-dimensional coordinates of a target pointlocated at a position in space, the method including steps of: providinga coordinate measurement device that includes a first light source thatproduces a first light at a first wavelength, a second light source thatproduces a second light at a second wavelength different than the firstwavelength, a fiber-optic coupler that includes at least a first port, asecond port, and a third port, an optical system that includes anoptical element achromatic at the first wavelength and the secondwavelength, a first motor, a second motor, a first angle measuringdevice, a second angle measuring device, a distance meter, and aprocessor; coupling a first portion of the first light into the firstport; coupling a second portion of the second light into the secondport; transmitting a third light from the third port, the third lightcontaining a portion of the first portion and a portion of the secondportion; transmitting a portion of the third light through the opticalsystem out of the coordinate measurement device as a first beam of lightcollimated at the first wavelength and the second wavelength; directingthe first beam of light in a first direction, the first directiondetermined by a first angle of rotation about a first axis and a secondangle of rotation about a second axis, the first angle of rotationproduced by the first motor and the second angle of rotation produced bythe second motor; measuring the first angle of rotation with the firstangle measuring device; measuring the second angle of rotation with thesecond angle measuring device; reflecting a portion of the first beamfrom the target point as a second beam; measuring a first distance fromthe coordinate measurement device to the target point with the distancemeter, the measured distance based at least in part on a third portionof the second beam of light received by a first optical detector;determining three-dimensional coordinates of the target point based atleast in part on the first distance, the first angle of rotation, andthe second angle of rotation; and storing the determinedthree-dimensional coordinates.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, exemplary embodiments are shown whichshould not be construed to be limiting regarding the entire scope of thedisclosure, and wherein the elements are numbered alike in severalFIGURES:

FIG. 1 is a perspective view of a laser tracker system with aretroreflector target in accordance with an embodiment of the presentinvention;

FIG. 2 is a perspective view of a laser tracker system with a six-DOFtarget in accordance with an embodiment of the present invention;

FIG. 3 is a block diagram describing elements of laser tracker opticsand electronics in accordance with an embodiment of the presentinvention;

FIGS. 4A and 4B show two types of prior art afocal beam expanders;

FIG. 5 shows a prior art fiber-optic beam launch;

FIG. 6A-D are schematic figures that shows four types of prior artposition detector assemblies, and FIGS. 6E, 6F are schematic figuresshowing position detector assemblies according to embodiments of thepresent invention;

FIG. 7 is a block diagram of electrical and electro-optical elementswithin a prior art ADM;

FIGS. 8A and 8B are schematic figures showing fiber-optic elementswithin a prior art fiber-optic network;

FIG. 8C is a schematic figure showing fiber-optic elements within afiber-optic network in accordance with an embodiment of the presentinvention;

FIG. 9 is an exploded view of a prior art laser tracker;

FIG. 10 is a cross-sectional view of a prior art laser tracker;

FIG. 11 is a block diagram of the computing and communication elementsof a laser tracker in accordance with an embodiment of the presentinvention;

FIG. 12A is a block diagram of elements in a laser tracker that uses asingle wavelength according to an embodiment of the present invention;

FIG. 12B is a block diagram of elements in a laser tracker that uses asingle wavelength according to an embodiment of the present invention;

FIG. 13 is a block diagram of elements in a laser tracker with six DOFcapability according to an embodiment of the present invention;

FIGS. 14A-D are block diagrams of elements in a laser tracker having sixDOF capability according to an embodiment of the present invention;

FIG. 15 is a block diagram of elements in a laser tracker according toan embodiment of the present invention;

FIG. 16 is a schematic diagram showing elements within a fiber-opticassembly according to an embodiment of the present invention;

FIG. 17 is a block diagram of elements in a laser tracker having six DOFcapability according to an embodiment of the present invention;

FIG. 18 is a flowchart of a method for measuring three-dimensionalcoordinates of a retroreflector target according to embodiments of thepresent invention;

FIG. 19 is flowchart of a method for measuring three-dimensionalcoordinates of a retroreflector target according to embodiments of thepresent invention; and

FIGS. 20A, 20B, and 20C are schematic illustrations of three lensarchitectures configured for collimating light of several wavelengthsaccording to an embodiment.

DETAILED DESCRIPTION

An exemplary laser tracker system 5 illustrated in FIG. 1 includes alaser tracker 10, a retroreflector target 26, an optional auxiliary unitprocessor 50, and an optional auxiliary computer 60. An exemplarygimbaled beam-steering mechanism 12 of laser tracker 10 comprises azenith carriage 14 mounted on an azimuth base 16 and rotated about anazimuth axis 20. A payload 15 is mounted on the zenith carriage 14 androtated about a zenith axis 18. Zenith axis 18 and azimuth axis 20intersect orthogonally, internally to tracker 10, at gimbal point 22,which is typically the origin for distance measurements. A laser beam 46virtually passes through the gimbal point 22 and is pointed orthogonalto zenith axis 18. In other words, laser beam 46 is approximatelyperpendicular to any plane parallel to both the zenith axis 18 and theazimuth axis 20. Outgoing laser beam 46 is pointed in the desireddirection by rotation of payload 15 about zenith axis 18 and by rotationof zenith carriage 14 about azimuth axis 20. A zenith angular encoder,internal to the tracker, is attached to a zenith mechanical axis alignedto the zenith axis 18. An azimuth angular encoder, internal to thetracker, is attached to an azimuth mechanical axis aligned to theazimuth axis 20. The zenith and azimuth angular encoders measure thezenith and azimuth angles of rotation to relatively high accuracy.Outgoing laser beam 46 travels to the retroreflector target 26, whichmight be, for example, a spherically mounted retroreflector (SMR) asdescribed above. By measuring the radial distance between gimbal point22 and retroreflector 26, the rotation angle about the zenith axis 18,and the rotation angle about the azimuth axis 20, the position ofretroreflector 26 is found within the spherical coordinate system of thetracker.

Outgoing laser beam 46 may include one or more laser wavelengths, asdescribed hereinafter. For the sake of clarity and simplicity, asteering mechanism of the sort shown in FIG. 1 is assumed in thefollowing discussion. However, other types of steering mechanisms arepossible. For example, it is possible to reflect a laser beam off amirror rotated about the azimuth and zenith axes. The techniquesdescribed herein are applicable, regardless of the type of steeringmechanism.

Magnetic nests 17 may be included on the laser tracker for resetting thelaser tracker to a “home” position for different sized SMRs—for example,1.5, ⅞, and ½ inch SMRs. An on-tracker retroreflector 19 may be used toreset the tracker to a reference distance. In addition, an on-trackermirror, not visible from the view of FIG. 1, may be used in combinationwith the on-tracker retroreflector to enable performance of aself-compensation, as described in U.S. Pat. No. 7,327,446, the contentsof which are incorporated by reference.

FIG. 2 shows an exemplary laser tracker system 7 that is like the lasertracker system 5 of FIG. 1 except that retroreflector target 26 isreplaced with a six-DOF probe 1000. In FIG. 1, other types ofretroreflector targets may be used. For example, a cateyeretroreflector, which is a glass retroreflector in which light focusesto a small spot of light on a reflective rear surface of the glassstructure, is sometimes used.

FIG. 3 is a block diagram showing optical and electrical elements in alaser tracker embodiment. It shows elements of a laser tracker that emittwo wavelengths of light—a first wavelength for an ADM and a secondwavelength for a visible pointer and for tracking. The visible pointerenables the user to see the position of the laser beam spot emitted bythe tracker. The two different wavelengths are combined using afree-space beam splitter. Electrooptic (EO) system 100 includes visiblelight source 110, isolator 115, optional first fiber launch 170,optional interferometer (IFM) 120, beam expander 140, first beamsplitter 145, position detector assembly 150, second beam splitter 155,ADM 160, and second fiber launch 170.

Visible light source 110 may be a laser, superluminescent diode, orother light emitting device. The isolator 115 may be a Faraday isolator,attenuator, or other device capable of reducing the light that reflectsback into the light source. Optional IFM may be configured in a varietyof ways. As a specific example of a possible implementation, the IFM mayinclude a beam splitter 122, a retroreflector 126, quarter waveplates124, 130, and a phase analyzer 128. The visible light source 110 maylaunch the light into free space, the light then traveling in free spacethrough the isolator 115, and optional IFM 120. Alternatively, theisolator 115 may be coupled to the visible light source 110 by a fiberoptic cable. In this case, the light from the isolator may be launchedinto free space through the first fiber-optic launch 170, as discussedherein below with reference to FIG. 5.

Beam expander 140 may be set up using a variety of lens configurations,but two commonly used prior-art configurations are shown in FIGS. 4A,4B. FIG. 4A shows a configuration 140A based on the use of a negativelens 141A and a positive lens 142A. A beam of collimated light 220Aincident on the negative lens 141A emerges from the positive lens 142Aas a larger beam of collimated light 230A. FIG. 4B shows a configuration140B based on the use of two positive lenses 141B, 142B. A beam ofcollimated light 220B incident on a first positive lens 141B emergesfrom a second positive lens 142B as a larger beam of collimated light230B. Of the light leaving the beam expander 140, a small amountreflects off the beam splitters 145, 155 on the way out of the trackerand is lost. That part of the light that passes through the beamsplitter 155 is combined with light from the ADM 160 to form a compositebeam of light 188 that leaves that laser tracker and travels to theretroreflector 90.

In an embodiment, the ADM 160 includes a light source 162, ADMelectronics 164, a fiber network 166, an interconnecting electricalcable 165, and interconnecting optical fibers 168, 169, 184, 186. ADMelectronics send electrical modulation and bias voltages to light source162, which may, for example, be a distributed feedback laser thatoperates at a wavelength of approximately 1550 nm. In an embodiment, thefiber network 166 may be the prior art fiber-optic network 420A shown inFIG. 8A. In this embodiment, light from the light source 162 in FIG. 3travels over the optical fiber 184, which is equivalent to the opticalfiber 432 in FIG. 8A.

The fiber network of FIG. 8A includes a first fiber coupler 430, asecond fiber coupler 436, and low-transmission reflectors 435, 440. Thelight travels through the first fiber coupler 430 and splits between twopaths, the first path through optical fiber 433 to the second fibercoupler 436 and the second path through optical fiber 422 and fiberlength equalizer 423. Fiber length equalizer 423 connects to fiberlength 168 in FIG. 3, which travels to the reference channel of the ADMelectronics 164. The purpose of fiber length equalizer 423 is to matchthe length of optical fibers traversed by light in the reference channelto the length of optical fibers traversed by light in the measurechannel. Matching the fiber lengths in this way reduces ADM errorscaused by changes in the ambient temperature. Such errors may arisebecause the effective optical path length of an optical fiber is equalto the average index of refraction of the optical fiber times the lengthof the fiber. Since the index of refraction of the optical fibersdepends on the temperature of the fiber, a change in the temperature ofthe optical fibers causes changes in the effective optical path lengthsof the measure and reference channels. If the effective optical pathlength of the optical fiber in the measure channel changes relative tothe effective optical path length of the optical fiber in the referencechannel, the result will be an apparent shift in the position of theretroreflector target 90, even if the retroreflector target 90 is keptstationary. To get around this problem, two steps are taken. First, thelength of the fiber in the reference channel is matched, as nearly aspossible, to the length of the fiber in the measure channel. Second, themeasure and reference fibers are routed side by side to the extentpossible to ensure that the optical fibers in the two channels seenearly the same changes in temperature.

The light travels through the second fiber optic coupler 436 and splitsinto two paths, the first path to the low-reflection fiber terminator440 and the second path to optical fiber 438, from which it travels tooptical fiber 186 in FIG. 3. The light on optical fiber 186 travelsthrough to the second fiber launch 170.

In an embodiment, fiber launch 170 is shown in prior art FIG. 5. Thelight from optical fiber 186 of FIG. 3 goes to fiber 172 in FIG. 5. Thefiber launch 170 includes optical fiber 172, ferrule 174, and lens 176.The optical fiber 172 is attached to ferrule 174, which is stablyattached to a structure within the laser tracker 10. If desired, the endof the optical fiber may be polished at an angle to reduce backreflections. The light 250 emerges from the core of the fiber, which maybe a single mode optical fiber with a diameter of between 4 and 12micrometers, depending on the wavelength of the light being used and theparticular type of optical fiber. The light 250 diverges at an angle andintercepts lens 176, which collimates it. The method of launching andreceiving an optical signal through a single optical fiber in an ADMsystem was described in reference to FIG. 3 in patent '758.

Referring to FIG. 3, the beam splitter 155 may be a dichroic beamsplitter, which transmits different wavelengths than it reflects. In anembodiment, the light from the ADM 160 reflects off dichroic beamsplitter 155 and combines with the light from the visible laser 110,which is transmitted through the dichroic beam splitter 155. Thecomposite beam of light 188 travels out of the laser tracker toretroreflector 90 as a first beam, which returns a portion of the lightas a second beam. That portion of the second beam that is at the ADMwavelength reflects off the dichroic beam splitter 155 and returns tothe second fiber launch 170, which couples the light back into theoptical fiber 186.

In an embodiment, the optical fiber 186 corresponds to the optical fiber438 in FIG. 8A. The returning light travels from optical fiber 438through the second fiber coupler 436 and splits between two paths. Afirst path leads to optical fiber 424 that, in an embodiment,corresponds to optical fiber 169 that leads to the measure channel ofthe ADM electronics 164 in FIG. 3. A second path leads to optical fiber433 and then to the first fiber coupler 430. The light leaving the firstfiber coupler 430 splits between two paths, a first path to the opticalfiber 432 and a second path to the low reflectance termination 435. Inan embodiment, optical fiber 432 corresponds to the optical fiber 184,which leads to the light source 162 in FIG. 3. In most cases, the lightsource 162 contains a built-in Faraday isolator that minimizes theamount of light that enters the light source from optical fiber 432.Excessive light fed into a laser in the reverse direction candestabilize the laser.

The light from the fiber network 166 enters ADM electronics 164 throughoptical fibers 168, 169. An embodiment of prior art ADM electronics isshown in FIG. 7. Optical fiber 168 in FIG. 3 corresponds to opticalfiber 3232 in FIG. 7, and optical fiber 169 in FIG. 3 corresponds tooptical fiber 3230 in FIG. 7. Referring now to FIG. 7, ADM electronics3300 includes a frequency reference 3302, a synthesizer 3304, a measuredetector 3306, a reference detector 3308, a measure mixer 3310, areference mixer 3312, conditioning electronics 3314, 3316, 3318, 3320, adivide-by-N prescaler 3324, and an analog-to-digital converter (ADC)3322. The frequency reference, which might be an oven-controlled crystaloscillator (OCXO), for example, sends a reference frequency f_(REF),which might be 10 MHz, for example, to the synthesizer, which generatestwo electrical signals—one signal at a frequency f_(RF) and two signalsat frequency f_(LO). The signal f_(RF) goes to the light source 3102,which corresponds to the light source 162 in FIG. 3. The two signals atfrequency f_(LO) go to the measure mixer 3310 and the reference mixer3312. The light from optical fibers 168, 169 in FIG. 3 appear on fibers3232, 3230 in FIG. 7, respectively, and enter the reference and measurechannels, respectively. Reference detector 3308 and measure detector3306 convert the optical signals into electrical signals. These signalsare conditioned by electrical components 3316, 3314, respectively, andare sent to mixers 3312, 3310, respectively. The mixers produce afrequency f_(IF) equal to the absolute value of f_(LO)−f_(RF). Thesignal f_(RF) may be a relatively high frequency, for example, 2 GHz,while the signal f_(IF) may have a relatively low frequency, forexample, 10 kHz.

The reference frequency f_(REF) is sent to the prescaler 3324, whichdivides the frequency by an integer value. For example, a frequency of10 MHz might be divided by a 40 to obtain an output frequency of 250kHz. In this example, the 10 kHz signals entering the ADC 3322 would besampled at a rate of 250 kHz, thereby producing 25 samples per cycle.The signals from the ADC 3322 are sent to a data processor 3400, whichmight, for example, be one or more digital signal processor (DSP) unitslocated in ADM electronics 164 of FIG. 3.

The method for extracting a distance is based on the calculation ofphase of the ADC signals for the reference and measure channels. Thismethod is described in detail in U.S. Pat. No. 7,701,559 ('559) toBridges et al., the contents of which are herein incorporated byreference. Calculation includes use of equations (1)-(8) of patent '559.In addition, when the ADM first begins to measure a retroreflector, thefrequencies generated by the synthesizer are changed some number oftimes (for example, three times), and the possible ADM distancescalculated in each case. By comparing the possible ADM distances foreach of the selected frequencies, an ambiguity in the ADM measurement isremoved. The equations (1)-(8) of patent '559 combined withsynchronization methods described with respect to FIG. 5 of patent '559and the Kalman filter methods described in patent '559 enable the ADM tomeasure a moving target. In other embodiments, other methods ofobtaining absolute distance measurements, for example, by using pulsedtime-of-flight rather than phase differences, may be used.

The part of the return light beam 190 that passes through the beamsplitter 155 arrives at the beam splitter 145, which sends part of thelight to the beam expander 140 and another part of the light to theposition detector assembly 150. The light emerging from the lasertracker 10 or EO system 100 may be thought of as a first beam and theportion of that light reflecting off the retroreflector 90 or 26 as asecond beam. Portions of the reflected beam are sent to differentfunctional elements of the EO system 100. For example, a first portionmay be sent to a distance meter such as an ADM 160 in FIG. 3. A secondportion may be sent to a position detector assembly 150. In some cases,a third portion may be sent to other functional units such as anoptional interferometer (120). It is important to understand that,although, in the example of FIG. 3, the first portion and the secondportion of the second beam are sent to the distance meter and theposition detector after reflecting off beam splitters 155 and 145,respectively, it would have been possible to transmit, rather thanreflect, the light onto a distance meter or position detector.

Four examples of prior art position detector assemblies 150A-150D areshown in FIGS. 6A-D. FIG. 6A depicts the simplest implementation, withthe position detector assembly including a position sensor 151 mountedon a circuit board 152 that obtains power from and returns signals toelectronics box 350, which may represent electronic processingcapability at any location within the laser tracker 10, auxiliary unit50, or external computer 60. FIG. 6B includes an optical filter 154 thatblocks unwanted optical wavelengths from reaching the position sensor151. The unwanted optical wavelengths may also be blocked, for example,by coating the beam splitter 145 or the surface of the position sensor151 with an appropriate film. FIG. 6C includes a lens 153 that reducesthe size of the beam of light. FIG. 6D includes both an optical filter154 and a lens 153.

FIG. 6E shows a position detector assembly according to embodiments ofthe present invention that includes an optical conditioner 149E. Opticalconditioner contains a lens 153 and may also contain optional wavelengthfilter 154. In addition, it includes at least one of a diffuser 156 anda spatial filter 157. As explained hereinabove, a popular type ofretroreflector is the cube-corner retroreflector. One type of cubecorner retroreflector is made of three mirrors, each joined at rightangles to the other two mirrors. Lines of intersection at which thesethree mirrors are joined may have a finite thickness in which light isnot perfectly reflected back to the tracker. The lines of finitethickness are diffracted as they propagate so that upon reaching theposition detector they may not appear exactly the same as at theposition detector. However, the diffracted light pattern will generallydepart from perfect symmetry. As a result, the light that strikes theposition detector 151 may have, for example, dips or rises in opticalpower (hot spots) in the vicinity of the diffracted lines. Because theuniformity of the light from the retroreflector may vary fromretroreflector to retroreflector and also because the distribution oflight on the position detector may vary as the retroreflector is rotatedor tilted, it may be advantageous to include a diffuser 156 to improvethe smoothness of the light that strikes the position detector 151. Itmight be argued that, because an ideal position detector should respondto a centroid and an ideal diffuser should spread a spot symmetrically,there should be no effect on the resulting position given by theposition detector. However, in practice the diffuser is observed toimprove performance of the position detector assembly, probably becausethe effects of nonlinearities (imperfections) in the position detector151 and the lens 153. Cube corner retroreflectors made of glass may alsoproduce non-uniform spots of light at the position detector 151.Variations in a spot of light at a position detector may be particularlyprominent from light reflected from cube corners in six-DOF targets, asmay be understood more clearly from commonly assigned U.S. patentapplication Ser. No. 13/370,339 filed Feb. 10, 2012, and Ser. No.13/407,983, filed Feb. 29, 2012, the contents of which are incorporatedby reference. In an embodiment, the diffuser 156 is a holographicdiffuser. A holographic diffuser provides controlled, homogeneous lightover a specified diffusing angle. In other embodiments, other types ofdiffusers such as ground glass or “opal” diffusers are used.

The purpose of the spatial filter 157 of the position detector assembly150E is to block ghost beams that may be the result, for example, ofunwanted reflections off optical surfaces, from striking the positiondetector 151. A spatial filter includes a plate 157 that has anaperture. By placing the spatial filter 157 a distance away from thelens equal approximately to the focal length of the lens, the returninglight 243E passes through the spatial filter when it is near itsnarrowest—at the waist of the beam. Beams that are traveling at adifferent angle, for example, as a result of reflection of an opticalelement strike the spatial filter away from the aperture and are blockedfrom reaching the position detector 151. An example is shown in FIG. 6E,where an unwanted ghost beam 244E reflects off a surface of the beamsplitter 145 and travels to spatial filter 157, where it is blocked.Without the spatial filter, the ghost beam 244E would have interceptedthe position detector 151, thereby causing the position of the beam 243Eon the position detector 151 to be incorrectly determined. Even a weakghost beam may significantly change the position of the centroid on theposition detector 151 if the ghost beam is located a relatively largedistance from the main spot of light.

A retroreflector of the sort discussed here, a cube corner or a cateyeretroreflector, for example, has the property of reflecting a ray oflight that enters the retroreflector in a direction parallel to theincident ray. In addition, the incident and reflected rays aresymmetrically placed about the point of symmetry of the retroreflector.For example, in an open-air cube corner retroreflector, the point ofsymmetry of the retroreflector is the vertex of the cube corner. In aglass cube corner retroreflector, the point of symmetry is also thevertex, but one must consider the bending of the light at the glass-airinterface in this case. In a cateye retroreflector having an index ofrefraction of 2.0, the point of symmetry is the center of the sphere. Ina cateye retroreflector made of two glass hemispheres symmetricallyseated on a common plane, the point of symmetry is a point lying on theplane and at the spherical center of each hemisphere. The main point isthat, for the type of retroreflectors ordinarily used with lasertrackers, the light returned by a retroreflector to the tracker isshifted to the other side of the vertex relative to the incident laserbeam.

This behavior of a retroreflector 90 in FIG. 3 is the basis for thetracking of the retroreflector by the laser tracker. The position sensorhas on its surface an ideal retrace point. The ideal retrace point isthe point at which a laser beam sent to the point of symmetry of aretroreflector (e.g., the vertex of the cube corner retroreflector in anSMR) will return. Usually the retrace point is near the center of theposition sensor. If the laser beam is sent to one side of theretroreflector, it reflects back on the other side and appears off theretrace point on the position sensor. By noting the position of thereturning beam of light on the position sensor, the control system ofthe laser tracker 10 can cause the motors to move the light beam towardthe point of symmetry of the retroreflector.

If the retroreflector is moved transverse to the tracker at a constantvelocity, the light beam at the retroreflector will strike theretroreflector (after transients have settled) a fixed offset distancefrom the point of symmetry of the retroreflector. The laser trackermakes a correction to account for this offset distance at theretroreflector based on scale factor obtained from controlledmeasurements and based on the distance from the light beam on theposition sensor to the ideal retrace point.

As explained hereinabove, the position detector performs two importantfunctions—enabling tracking and correcting measurements to account forthe movement of the retroreflector. The position sensor within theposition detector may be any type of device capable of measuring aposition. For example, the position sensor might be a position sensitivedetector or a photosensitive array. The position sensitive detectormight be lateral effect detector or a quadrant detector, for example.The photosensitive array might be a CMOS or CCD array, for example.

In an embodiment, the return light that does not reflect off beamsplitter 145 passes through beam expander 140, thereby becoming smaller.In another embodiment, the positions of the position detector and thedistance meter are reversed so that the light reflected by the beamsplitter 145 travels to the distance meter and the light transmitted bythe beam splitter travels to the position detector.

The light continues through optional IFM, through the isolator and intothe visible light source 110. At this stage, the optical power should besmall enough so that it does not destabilize the visible light source110.

In an embodiment, the light from visible light source 110 is launchedthrough a beam launch 170 of FIG. 5. The fiber launch may be attached tothe output of light source 110 or a fiber optic output of the isolator115.

In an embodiment, the fiber network 166 of FIG. 3 is prior art fibernetwork 420B of FIG. 8B. Here the optical fibers 184, 186, 168, 169 ofFIG. 3 correspond to optical fibers 443, 444, 424, 422 of FIG. 8B. Thefiber network of FIG. 8B is like the fiber network of FIG. 8A exceptthat the fiber network of FIG. 8B has a single fiber coupler instead oftwo fiber couplers. The advantage of FIG. 8B over FIG. 8A is simplicity;however, FIG. 8B is more likely to have unwanted optical backreflections entering the optical fibers 422 and 424.

In an embodiment, the fiber network 166 of FIG. 3 is fiber network 420Cof FIG. 8C. Here the optical fibers 184, 186, 168, 169 of FIG. 3correspond to optical fibers 447, 455, 423, 424 of FIG. 8C. The fibernetwork 420C includes a first fiber coupler 445 and a second fibercoupler 451. The first fiber coupler 445 is a 2×2 coupler having twoinput ports and two output ports. Couplers of this type are usually madeby placing two fiber cores in close proximity and then drawing thefibers while heated. In this way, evanescent coupling between the fiberscan split off a desired fraction of the light to the adjacent fiber. Thesecond fiber coupler 451 is of the type called a circulator. It hasthree ports, each having the capability of transmitting or receivinglight, but only in the designated direction. For example, the light onoptical fiber 448 enters port 453 and is transported toward port 454 asindicated by the arrow. At port 454, light may be transmitted to opticalfiber 455. Similarly, light traveling on port 455 may enter port 454 andtravel in the direction of the arrow to port 456, where some light maybe transmitted to the optical fiber 424. If only three ports are needed,then the circulator 451 may suffer less losses of optical power than the2×2 coupler. On the other hand, a circulator 451 may be more expensivethan a 2×2 coupler, and it may experience polarization mode dispersion,which can be problematic in some situations.

FIGS. 9 and 10 show exploded and cross sectional views, respectively, ofa prior art laser tracker 2100, which is depicted in FIGS. 2 and 3 ofU.S. Published Patent Application No. 2010/0128259 to Bridges et al.,incorporated by reference. Azimuth assembly 2110 includes post housing2112, azimuth encoder assembly 2120, lower and upper azimuth bearings2114A, 2114B, azimuth motor assembly 2125, azimuth slip ring assembly2130, and azimuth circuit boards 2135.

The purpose of azimuth encoder assembly 2120 is to accurately measurethe angle of rotation of yoke 2142 with respect to the post housing2112. Azimuth encoder assembly 2120 includes encoder disk 2121 andread-head assembly 2122. Encoder disk 2121 is attached to the shaft ofyoke housing 2142, and read head assembly 2122 is attached to postassembly 2110. Read head assembly 2122 comprises a circuit board ontowhich one or more read heads are fastened. Laser light sent from readheads reflect off fine grating lines on encoder disk 2121. Reflectedlight picked up by detectors on encoder read head(s) is processed tofind the angle of the rotating encoder disk in relation to the fixedread heads.

Azimuth motor assembly 2125 includes azimuth motor rotor 2126 andazimuth motor stator 2127. Azimuth motor rotor comprises permanentmagnets attached directly to the shaft of yoke housing 2142. Azimuthmotor stator 2127 comprises field windings that generate a prescribedmagnetic field. This magnetic field interacts with the magnets ofazimuth motor rotor 2126 to produce the desired rotary motion. Azimuthmotor stator 2127 is attached to post frame 2112.

Azimuth circuit boards 2135 represent one or more circuit boards thatprovide electrical functions required by azimuth components such as theencoder and motor. Azimuth slip ring assembly 2130 includes outer part2131 and inner part 2132. In an embodiment, wire bundle 2138 emergesfrom auxiliary unit processor 50. Wire bundle 2138 may carry power tothe tracker or signals to and from the tracker. Some of the wires ofwire bundle 2138 may be directed to connectors on circuit boards. In theexample shown in FIG. 10, wires are routed to azimuth circuit board2135, encoder read head assembly 2122, and azimuth motor assembly 2125.Other wires are routed to inner part 2132 of slip ring assembly 2130.Inner part 2132 is attached to post assembly 2110 and consequentlyremains stationary. Outer part 2131 is attached to yoke assembly 2140and consequently rotates with respect to inner part 2132. Slip ringassembly 2130 is designed to permit low impedance electrical contact asouter part 2131 rotates with respect to the inner part 2132.

Zenith assembly 2140 comprises yoke housing 2142, zenith encoderassembly 2150, left and right zenith bearings 2144A, 2144B, zenith motorassembly 2155, zenith slip ring assembly 2160, and zenith circuit board2165.

The purpose of zenith encoder assembly 2150 is to accurately measure theangle of rotation of payload frame 2172 with respect to yoke housing2142. Zenith encoder assembly 2150 comprises zenith encoder disk 2151and zenith read-head assembly 2152. Encoder disk 2151 is attached topayload housing 2142, and read head assembly 2152 is attached to yokehousing 2142. Zenith read head assembly 2152 comprises a circuit boardonto which one or more read heads are fastened. Laser light sent fromread heads reflect off fine grating lines on encoder disk 2151.Reflected light picked up by detectors on encoder read head(s) isprocessed to find the angle of the rotating encoder disk in relation tothe fixed read heads.

Zenith motor assembly 2155 comprises azimuth motor rotor 2156 andazimuth motor stator 2157. Zenith motor rotor 2156 comprises permanentmagnets attached directly to the shaft of payload frame 2172. Zenithmotor stator 2157 comprises field windings that generate a prescribedmagnetic field. This magnetic field interacts with the rotor magnets toproduce the desired rotary motion. Zenith motor stator 2157 is attachedto yoke frame 2142.

Zenith circuit board 2165 represents one or more circuit boards thatprovide electrical functions required by zenith components such as theencoder and motor. Zenith slip ring assembly 2160 comprises outer part2161 and inner part 2162. Wire bundle 2168 emerges from azimuth outerslip ring 2131 and may carry power or signals. Some of the wires of wirebundle 2168 may be directed to connectors on circuit board. In theexample shown in FIG. 10, wires are routed to zenith circuit board 2165,zenith motor assembly 2150, and encoder read head assembly 2152. Otherwires are routed to inner part 2162 of slip ring assembly 2160. Innerpart 2162 is attached to yoke frame 2142 and consequently rotates inazimuth angle only, but not in zenith angle. Outer part 2161 is attachedto payload frame 2172 and consequently rotates in both zenith andazimuth angles. Slip ring assembly 2160 is designed to permit lowimpedance electrical contact as outer part 2161 rotates with respect tothe inner part 2162. Payload assembly 2170 includes a main opticsassembly 2180 and a secondary optics assembly 2190.

FIG. 11 is a block diagram depicting a dimensional measurementelectronics processing system 1500 that includes a laser trackerelectronics processing system 1510, peripheral elements 1582, 1584,1586, computer 1590, and other networked components 1600, representedhere as a cloud. Exemplary laser tracker electronics processing system1510 includes a master processor 1520, payload functions electronics1530, azimuth encoder electronics 1540, zenith encoder electronics 1550,display and user interface (UI) electronics 1560, removable storagehardware 1565, radio frequency identification (RFID) electronics, and anantenna 1572. The payload functions electronics 1530 includes a numberof subfunctions including the six-DOF electronics 1531, the cameraelectronics 1532, the ADM electronics 1533, the position detector (PSD)electronics 1534, and the level electronics 1535. Most of thesubfunctions have at least one processor unit, which might be a digitalsignal processor (DSP) or field programmable gate array (FPGA), forexample. The electronics units 1530, 1540, and 1550 are separated asshown because of their location within the laser tracker. In anembodiment, the payload functions 1530 are located in the payload 2170of FIGS. 9, 10, while the azimuth encoder electronics 1540 is located inthe azimuth assembly 2110 and the zenith encoder electronics 1550 islocated in the zenith assembly 2140.

Many types of peripheral devices are possible, but here three suchdevices are shown: a temperature sensor 1582, a six-DOF probe 1584, anda personal digital assistant, 1586, which might be a smart phone, forexample. The laser tracker may communicate with peripheral devices in avariety of means, including wireless communication over the antenna1572, by means of a vision system such as a camera, and by means ofdistance and angular readings of the laser tracker to a cooperativetarget such as the six-DOF probe 1584.

In an embodiment, a separate communications bus goes from the masterprocessor 1520 to each of the electronics units 1530, 1540, 1550, 1560,1565, and 1570. Each communications line may have, for example, threeserial lines that include the data line, clock line, and frame line. Theframe line indicates whether or not the electronics unit should payattention to the clock line. If it indicates that attention should begiven, the electronics unit reads the current value of the data line ateach clock signal. The clock signal may correspond, for example, to arising edge of a clock pulse. In an embodiment, information istransmitted over the data line in the form of a packet. In anembodiment, each packet includes an address, a numeric value, a datamessage, and a checksum. The address indicates where, within theelectronics unit, the data message is to be directed. The location may,for example, correspond to a processor subroutine within the electronicsunit. The numeric value indicates the length of the data message. Thedata message contains data or instructions for the electronics unit tocarry out. The checksum is a numeric value that is used to minimize thechance that errors are transmitted over the communications line.

In an embodiment, the master processor 1520 sends packets of informationover bus 1610 to payload functions electronics 1530, over bus 1611 toazimuth encoder electronics 1540, over bus 1612 to zenith encoderelectronics 1550, over bus 1613 to display and UI electronics 1560, overbus 1614 to removable storage hardware 1565, and over bus 1616 to RFIDand wireless electronics 1570.

In an embodiment, master processor 1520 also sends a synch(synchronization) pulse over the synch bus 1630 to each of theelectronics units at the same time. The synch pulse provides a way ofsynchronizing values collected by the measurement functions of the lasertracker. For example, the azimuth encoder electronics 1540 and thezenith electronics 1550 latch their encoder values as soon as the synchpulse is received. Similarly, the payload functions electronics 1530latch the data collected by the electronics contained within thepayload. The six-DOF, ADM, and position detector all latch data when thesynch pulse is given. In most cases, the camera and inclinometer collectdata at a slower rate than the synch pulse rate but may latch data atmultiples of the synch pulse period.

The azimuth encoder electronics 1540 and zenith encoder electronics 1550are separated from one another and from the payload electronics 1530 bythe slip rings 2130, 2160 shown in FIGS. 9, 10. This is why the buslines 1610, 1611, and 1612 are depicted as separate bus line in FIG. 11.

The laser tracker electronics processing system 1510 may communicatewith an external computer 1590, or it may provide computation, display,and UI functions within the laser tracker. The laser trackercommunicates with computer 1590 over communications link 1606, whichmight be, for example, and Ethernet line or a wireless connection. Thelaser tracker may also communicate with other elements 1600, representedby the cloud, over communications link 1602, which might include one ormore electrical cables, such as Ethernet cables, and one or morewireless connections. An example of an element 1600 is another threedimensional test instrument—for example, an articulated arm CMM, whichmay be relocated by the laser tracker. A communication link 1604 betweenthe computer 1590 and the elements 1600 may be wired (e.g., Ethernet) orwireless. An operator sitting on a remote computer 1590 may make aconnection to the Internet, represented by the cloud 1600, over anEthernet or wireless line, which in turn connects to the masterprocessor 1520 over an Ethernet or wireless line. In this way, a usermay control the action of a remote laser tracker.

Laser trackers today use one visible wavelength (usually red) and oneinfrared wavelength for the ADM. The red wavelength may be provided by afrequency stabilized helium-neon (HeNe) laser suitable for use in aninterferometer and also for use in providing a red pointer beam.Alternatively, the red wavelength may be provided by a diode laser thatserves just as a pointer beam. A disadvantage in using two light sourcesis the extra space and added cost required for the extra light sources,beam splitters, isolators, and other components. Another disadvantage inusing two light sources is that it is difficult to perfectly align thetwo light beams along the entire paths the beams travel. This may resultin a variety of problems including inability to simultaneously obtaingood performance from different subsystems that operate at differentwavelengths. A system that uses a single light source, therebyeliminating these disadvantages, is shown in opto-electronic system 500of FIG. 12A.

FIG. 12A includes a visible light source 110, an isolator 115, a fibernetwork 420, ADM electronics 530, a fiber launch 170, a beam splitter145, and a position detector 150. The visible light source 110 might be,for example, a red or green diode laser or a vertical cavity surfaceemitting laser (VCSEL). The isolator might be a Faraday isolator, anattenuator, or any other device capable of sufficiently reducing theamount of light fed back into the light source. The light from theisolator 115 travels into the fiber network 420, which in an embodimentis the fiber network 420A of FIG. 8A.

FIG. 12B shows an embodiment of an optoelectronic system 400 in which asingle wavelength of light is used but wherein modulation is achieved bymeans of electro-optic modulation of the light rather than by directmodulation of a light source. The optoelectronic system 400 includes avisible light source 110, an isolator 115, an electrooptic modulator410, ADM electronics 475, a fiber network 420, a fiber launch 170, abeam splitter 145, and a position detector 150. The visible light source110 may be, for example, a red or green laser diode. Laser light is sentthrough an isolator 115, which may be a Faraday isolator or anattenuator, for example. The isolator 115 may be fiber coupled at itsinput and output ports. The isolator 115 sends the light to theelectrooptic modulator 410, which modulates the light to a selectedfrequency, which may be up to 10 GHz or higher if desired. An electricalsignal 476 from ADM electronics 475 drives the modulation in theelectrooptic modulator 410. The modulated light from the electroopticmodulator 410 travels to the fiber network 420, which might be the fibernetwork 420A, 420B, 420C, or 420D discussed hereinabove. Some of thelight travels over optical fiber 422 to the reference channel of the ADMelectronics 475. Another portion of the light travels out of thetracker, reflects off retroreflector 90, returns to the tracker, andarrives at the beam splitter 145. A small amount of the light reflectsoff the beam splitter and travels to position detector 150, which hasbeen discussed hereinabove with reference to FIGS. 6A-F. A portion ofthe light passes through the beam splitter 145 into the fiber launch170, through the fiber network 420 into the optical fiber 424, and intothe measure channel of the ADM electronics 475. In general, the system500 of FIG. 12A can be manufactured for less money than system 400 ofFIG. 12B; however, the electro-optic modulator 410 may be able toachieve a higher modulation frequency, which can be advantageous in somesituations.

FIG. 13 shows an embodiment of a locator camera system 950 and anoptoelectronic system 900 in which an orientation camera is combinedwith the optoelectronic functionality of a 3D laser tracker to measuresix degrees of freedom. The optoelectronic system 900 includes a visiblelight source 905, an isolator 910, an optional electrooptic modulator410, ADM electronics 715, a fiber network 420, a fiber launch 170, abeam splitter 145, a position detector 150, a beam splitter 922, and anorientation camera 910. The light from the visible light source isemitted in optical fiber 980 and travels through isolator 910, which mayhave optical fibers coupled on the input and output ports. The light maytravel through the electrooptic modulator 410 modulated by an electricalsignal 716 from the ADM electronics 715. Alternatively, the ADMelectronics 715 may send an electrical signal over cable 717 to modulatethe visible light source 905. Some of the light entering the fibernetwork travels through the fiber length equalizer 423 and the opticalfiber 422 to enter the reference channel of the ADM electronics 715. Anelectrical signal 469 may optionally be applied to the fiber network 420to provide a switching signal to a fiber optic switch within the fibernetwork 420. A part of the light travels from the fiber network to thefiber launch 170, which sends the light on the optical fiber into freespace as light beam 982. A small amount of the light reflects off thebeamsplitter 145 and is lost. A portion of the light passes through thebeam splitter 145, through the beam splitter 922, and travels out of thetracker to six degree-of-freedom (DOF) device 4000. The six DOF device4000 may be a probe, a scanner, a projector, a sensor, or other device.

On its return path, the light from the six-DOF device 4000 enters theoptoelectronic system 900 and arrives at beamsplitter 922. Part of thelight is reflected off the beamsplitter 922 and enters the orientationcamera 910. The orientation camera 910 records the positions of somemarks placed on the retroreflector target. From these marks, theorientation angle (i.e., three degrees of freedom) of the six-DOF probeis found. The principles of the orientation camera are describedhereinafter in the present application and also in patent '758. Aportion of the light at beam splitter 145 travels through thebeamsplitter and is put onto an optical fiber by the fiber launch 170.The light travels to fiber network 420. Part of this light travels tooptical fiber 424, from which it enters the measure channel of the ADMelectronics 715.

The locator camera system 950 includes a camera 960 and one or morelight sources 970. The camera includes a lens system 962, aphotosensitive array 964, and a body 966. One use of the locator camerasystem 950 is to locate retroreflector targets in the work volume. Itdoes this by flashing the light source 970, which the camera picks up asa bright spot on the photosensitive array 964. A second use of thelocator camera system 950 is establish a coarse orientation of thesix-DOF device 4000 based on the observed location of a reflector spotor LED on the six-DOF device 4000. If two or more locator camera systemsare available on the laser tracker, the direction to each retroreflectortarget in the work volume may be calculated using the principles oftriangulation. If a single locator camera is located to pick up lightreflected along the optical axis of the laser tracker, the direction toeach retroreflector target may be found. If a single camera is locatedoff the optical axis of the laser tracker, then approximate directionsto the retroreflector targets may be immediately obtained from the imageon the photosensitive array. In this case, a more accurate direction toa target may be found by rotating the mechanical axes of the laser tomore than one direction and observing the change in the spot position onthe photosensitive array.

In an embodiment, the electrooptics module 176 includes a combination ofoptical components, such as beam splitters and waveplates, andoptoelectronic components, such as optical detectors and amplifiers, toseparate the phase difference d into quadrature components. Thesequadrature components include sin(d) 188 and cos(d) 190. An electricalcounter uses the quadrature components to count the number of complete360 degree shifts in the phase difference d. This number of counts (andpossibly a fraction of a count) is sent the counter 178, which keepstrack of the number of counts. This number of counts is sent over a line180 to a processor, which calculates a distance corresponding to thenumber of counts.

FIG. 14A shows an embodiment of an orientation camera 910, which may beused in the optoelectronic systems of FIGS. 18 and 19. The generalprinciples of the orientation camera are described in patent '758 andare generally adhered to in orientation camera 910. In an embodiment,the orientation camera 910 includes a body 1210, an afocal beam reducer1220, a magnifier 1240, a path length adjuster 1230, an actuatorassembly 1260, and a photosensitive array 1250. The afocal beam reducerincludes a positive lens 1222, a mirror 1223, and negative lenses 1224,1226. The afocal beam reducer has the property that a ray of light thatenters lens 1222 parallel to an optical axis—an axis that passes throughthe center of the lenses—emerges from lens 1226 also parallel to theoptical axis. The afocal beam reducer also has the property that animage has a constant size regardless of the distance from the lens to anobject. The magnifier 1240 includes a positive lens 1242, negativelenses 1244, 1248, and a mirror 1246. The magnifier has the samefunction as a microscope objective but is scaled to provide a largerimage. The photosensitive array 1250 may, for example, be a CMOS or CCDarray that converts the light that strikes it into an array of digitalvalues representing the irradiance of the light at each pixel of thephotosensitive array. The pattern of irradiance may reveal, for example,the marks on a six-DOF target. The path length adjuster 1230 includes aplatform 1231, two mirrors 1232, 1233, and a ball slide 1234. Themirrors 1232, 1233 are mounted on the platform 1231 so that when theplatform 1231 is moved, the distance between the afocal beam reducer1220 and the magnifier 1240 is changed. This change in distance isneeded to keep a clear image on the photosensitive array 1250 for achanging distance from the laser tracker to the target. The platform1231 is mounted on the ball slide 1234, which provides the platform withlow friction linear motion. In an embodiment, the actuator assembly 1260includes a motor 1261, a motor shaft 1262, a flexible coupling 1263, anadapter 1264, and a motor nut 1265. The motor nut 1265 is fixedlyattached to the adapter. As the threaded motor shaft 1262 is rotated bythe motor 1261, the motor nut 1265 is moved either farther from ornearer to the motor, depending on the direction of rotation of the motorshaft. The flexible coupler 1263, which is attached to the adapter 1264,allows the platform to move freely even if the motor shaft 1262 and theball slide 1234 are not parallel to one another.

In an embodiment, the orientation camera 910 provides constanttransverse magnification for different distances to the target. Heretransverse magnification is defined as the image size divided by theobject size. The lenses shown in FIG. 14A were selected to produce aconstant image size on the photosensitive array 1250 of 3 mm for anobject size of 13 mm. In this instance, the transverse magnification is3 mm/13 mm=0.23. This transverse magnification is held constant for atarget placed a distance from the tracker of between 0.5 meter and 30meters. This image size of 3 mm might be appropriate for a ¼inch CCD orCMOS array. In an embodiment, the transverse magnification is four timesthis amount, making it appropriate for a one inch CCD or CMOS array. Anorientation camera with this increased transverse magnification can beobtained in the same size body 1210, by changing the focal lengths andspacings of the three lenses in the magnifier 1240.

In an embodiment shown in FIG. 14A, the effective focal lengths of thethree lens elements 1222, 1224, and 1226 of the beam reducer 1220 are85.9 mm, −29.6 mm, and −7.2 mm, respectively. A virtual image is formedafter the light from the object passes through these three lenselements. For an object placed 0.5 meter from the laser tracker, thevirtual image 1229 has a size of 0.44 mm and is located 7 mm from thelens 1226. For an object placed 30 meters from the laser tracker, thevirtual image 1228 has a size of 0.44 mm and is located 1.8 mm from thelens 1224. The distance between the virtual image 1228 and the virtualimage 1129 is 39.8 mm, which means that the platform needs a maximumtravel range of half this amount, or 19.9 mm. The transversemagnification of the beam reducer 1220 is 0.44 mm/13 mm=0.034. Theeffective focal lengths of the three lens elements 1242, 1244, and 1228of the magnifier are 28.3 mm, −8.8 mm, and −8.8 mm, respectively. Thesize of the image at the photosensitive array 1250 is 3 mm for a targetlocated 0.5 meter from the laser tracker, 30 meters from the lasertracker, or any distance in between. The transverse magnification of themagnifier is 3 mm/0.44 mm=6.8. The overall transverse magnification ofthe orientation camera is 3 mm/13 mm=0.23. In another embodiment, thetransverse magnification of the magnifier is increased by a factor of 4to 4×6.8=27, thereby producing an overall transverse magnification of 12mm/13 mm=0.92 for any distance from 0.5 to 30 meters.

Another embodiment of an orientation camera is shown in FIGS. 14B-D.FIG. 14B is a side view of an orientation camera assembly 2750B. FIG.14C is a top view 2750C of a section A-A shown in FIG. 14B. FIG. 14D isa side sectional view 2750D of a section B-B of FIG. 14C. The path oflight beam 2755 is shown in each of the three figures. Light passesthrough a first collection of lenses 2760, reflects off mirror 2762,passes through lens 2764, reflects off mirrors 2766, 2768, passesthrough a section collection of lenses 2770, reflects off mirrors 2772,2774, and strikes photosensitive array 2776. The first collection oflenses 2760 and the lens 2764 form an afocal lens system. As explainedherein above, this means that a ray entering the first collection oflenses 2760 parallel to the optical axis will exit the lens 2764parallel to the optical axis. Because the retroreflector (not shown inFIGS. 14B-D is a finite distance from the laser tracker, the afocal lenssystem will produce a virtual image 2778 at some distance from the lens2764. This distance d from the lens 2764 will depend on the distancefrom the retroreflector from the laser tracker. For example, in anembodiment, the virtual image is about d=82 mm from the lens 2764 whenthe retroreflector is four meters from the tracker and about d=51 mmfrom the lens 2764 when the retroreflector is forty meters from thetracker. The second collection of lenses relays the virtual image 2778onto the photosensitive array. A motorized actuator 2780 adjusts theposition of mirrors 2766, 2768 in order to maintain the correct distancefrom the virtual image 2778 to the second collection of lenses 2770,thereby keeping the image on the photosensitive array 2776 in focus. Inan embodiment, the first collection of lenses 2755 has a combined focallength of 112 mm, the lens 2764 has a focal length of −5.18 mm, and thesecond collection of lenses 2770 has a combined focal length of about59.3 mm. The overall magnification of the system is approximately ⅛,which means that the size of the light pattern on the photosensitivearray 2776 is about one-eighth the size of the light pattern on theretroreflector. This is an example of a lens system that maintains aconstant magnification regardless of the distance from the laser trackerto the retroreflector.

Other combinations of lenses can be combined to make an orientationcamera having a constant transverse magnification. Furthermore, althoughhaving constant transverse magnification is helpful, other lens systemsare also useable. In general, the cameras of FIGS. 14A-D aredistinguished by having a zoom capability, a narrow field of view, andan alignment with the optical axis of the laser tracker.

FIG. 15 shows an embodiment of an optoelectronic system 700 in which twodifferent wavelengths of light are combined using a fiber optic coupler.The optoelectronic system 700 includes a first light source 705, asecond light source 750, a first isolator 710, a second isolator 755, anoptional electrooptic modulator 410, ADM electronics 715, a fibernetwork 720, a fiber launch 170, a beam splitter 145, and a positiondetector 150. The first light source 705 may be, for example, a diodelaser that operates at 780 nm. The second light source may be, forexample, a red or green diode laser. Light from the first light source705 is sent over an optical fiber 780 through an isolator 710, which maybe a Faraday isolator or an attenuator, for example. The isolator 710may be fiber coupled at its input and output ports. The isolator 710 maysend the light to an electrooptic modulator 410, which modulates thelight. If the electrooptic modulator 410 is used, an electrical signal716 from ADM electronics 715 drives the modulation in the electroopticmodulator 410. Alternatively, if the electrooptic modulator 410 isomitted, the ADM electronics 715 sends a modulation signal directly tothe light source 705. The light from the first light source travelsthrough optical fiber 781 to the fiber network 720. Some of the light isrouted through fiber length equalizer 423 and optical fiber 722 into thereference channel of the ADM electronics 715. Another part of the lighttravels out of the fiber network 720 through optical fiber 782 to thefiber launch, which sends the light beam 783 into free space. A smallamount of the light reflects off beam splitter 145 and is lost. The restof the light passes through beam splitter 145, travels to retroreflector90 as light beam 784, and travels back to the beam splitter 145 as lightbeam 786. Some of the light reflects off the beam splitter 145 andtravels to the position detector 150. Another part of the light passesthrough the fiber launch and is coupled back into the optical fiber 782.The light passes into the fiber network 720 and travels over opticalfiber 724 to the measure channel of the ADM electronics 715.

The second light source 750 sends a second beam of light onto opticalfiber 790, through isolator 755, through optical fiber 791 and intofiber network 720. An embodiment of fiber network 720 is shown in FIG.16. The light from optical fiber 1781 enters fiber network 720 at theinput port. The light travels through a first fiber coupler 1730. Partof the light travels through optical fiber 1722 and fiber lengthcompensator 1723 before entering the reference channel of ADMelectronics 715. Some of the light travels through a second fibercoupler 1740 and a third fiber coupler 1750 before passing out of thefiber network onto optical fiber 1782. The light from optical fiber 1791enters into the third fiber coupler 1750, where it is combined with thelight from optical fiber 1743 to form a composite light beam thattravels on optical fiber 1782. The ports attached to optical fibers 1781and 1791 are two input ports, and may be considered a first port and asecond port. The ports attached to optical fibers 1782 and 1755 areoutput ports and may be considered a third port and a fourth port. Theoptical coupler 1750 is a dichroic coupler because it is designed to usetwo wavelengths. After the composite light beam carried in optical fiber1782 travels out of the laser tracker and reflects off retroreflector90, it returns to the fiber network 720. The light from the first lightsource passes through the third fiber coupler 1750, the second fibercoupler 1740, and enters optical fiber 1724, which leads to the measurechannel of the ADM electronics 715. The light from the second lightsource returns to optical fiber 1791 and travels to isolator 755, whichkeeps it from entering the second light source 750.

The couplers 1730, 1740, and 1750 may be of the fused type. With thistype of optical coupler, two fiber core/cladding regions are broughtclose together and fused. Consequently, light between the cores isexchanged by evanescent coupling. In the case of two differentwavelengths, it is possible to design an evanescent coupling arrangementthat allows complete transmission of a first wavelength along theoriginal fiber and complete coupling of a second wavelength over to thesame fiber. In practical cases, it is not usually possible to obtain acomplete (100 percent) coupling of the light so that the fiber-opticcoupler provides lossless transmission. However, fiber-optic couplersthat provide good coupling for two or more different wavelengths may bepurchased and are readily available at common wavelengths such as 980nm, 1300 nm, and 1550 nm. In addition, fiber-optic couplers may bepurchased off-the-shelf for other wavelengths, including visiblewavelengths, and may be custom designed and manufactured for otherwavelengths. For example, in FIG. 16, it is possible to design fiberoptic coupler 1750 so that the first light at its first wavelengthtravels from optical fiber 1743 to optical fiber 7153 with low opticalloss. At the same time, the design can provide for a nearly completecoupling of the second light on optical fiber 1791 over to the opticalfiber 1782. Hence it is possible to transfer the first light and thesecond light through the fiber optic coupler and onto the same fiber1782 with low loss. It is possible to buy optical couplers that combinewavelengths that differ widely in wavelength. For example, it ispossible to buy a coupler that combines light at a wavelength of 1310 nmwith light at a wavelength of 660 nm. For propagation over longdistances with propagation of both wavelengths in a single transversemode while having relatively low loss of optical power duringpropagation through the optical fiber, it is generally required that thetwo wavelengths be relatively close together. For example, the twoselected wavelengths might be 633 nm and 780 nm, which are relativelyclose together in wavelength values and could be transmitted through asingle-mode optical fiber over a long distance without a high loss. Anadvantage of the architecture of the electrooptics assembly 700 is thatthe dichroic fiber coupler 1750 within the fiber network 720 is morecompact that a free space beam splitter. In addition, the dichroic fibercoupler ensures that the first light and the second light are very wellaligned without requiring any special optical alignment proceduresduring production.

FIG. 17 shows an embodiment of an electrooptic system 1900 similar tothe electrooptic system 900 of FIG. 13 except FIG. 17 contains two lightsources—a first light source 705 and a second light source 750. Thefirst light source 705, the second light source 750, the first isolator710, and the second isolator 755 of FIG. 17 are the same componentsshown in FIG. 15 and described hereinabove.

FIG. 18 shows a method 4010 for measuring three-dimensional coordinatesof a retroreflector target. A step 4015 is to provide a coordinatemeasurement device that includes a first light source that produces afirst light at a first wavelength, a second light source that produces asecond light at a second wavelength different than the first wavelength,a fiber-optic coupler that includes at least a first port, a secondport, and a third port, an optical system, a first motor, a secondmotor, a first angle measuring device, a second angle measuring device,a distance meter, and a processor. A step 4020 is to couple a firstportion (at a first wavelength) of the first light into the first port.A step 4025 is to couple a second portion (at a second wavelength) ofthe second light into the second port. A fourth step 4030 is to transmita third light from the third port, the third light containing a portionof the first portion and a portion of the second portion. A fifth step4035 is to transmit a portion of the third light through the opticalsystem and out of the coordinate measurement device as a first beam oflight. A step 4040 is to direct the first beam of light in a firstdirection, the first direction determined by a first angle of rotationabout a first axis and a second angle of rotation about a second axis,the first angle of rotation produced by the first motor and the secondangle of rotation produced by the second motor. A step 4045 is tomeasure the first angle of rotation with the first angle measuringdevice and to measure a second angle of rotation with the second anglemeasuring device. A step 4050 is to reflect a portion of the first beamfrom the retroreflector target as a second beam. A step 4055 is tomeasure a first distance from the coordinate measurement device to theretroreflector target with the distance meter, the measured distancebased at least in part on a third portion of the second beam of lightreceived by a first optical detector. A step 4060 is to determinethree-dimensional coordinates of the retroreflector target based atleast in part on the first distance, the first angle of rotation, andthe second angle of rotation. A step 4065 is to store the determinedthree-dimensional coordinates.

FIG. 19 shows a method 4110 for measuring three-dimensional coordinatesof a retroreflector target. A step 4115 is to provide a positiondetector assembly, the position detector assembly including a positiondetector. A step 4120 is to send a fourth portion of the second beamonto the position detector. A step 4125, which follows from the step Aof FIG. 18, is to obtain a first signal from the position detector, thefirst signal responsive to the position of the fourth portion on theposition detector. A step 4130 is to send a second signal to the firstmotor and sending a third signal to the second motor, the second signaland the third signal based at least in part on the first signal. A step4135 is to adjust the first direction of the first beam to the positionin space of the retroreflector target. The procedure terminates at stepB.

In an embodiment, light of multiple wavelengths is sent out of thetracker collimated, which is to say that the rays of light exiting thetracker are nearly parallel. In some cases, if light of differentwavelengths pass through different lens elements before being combined,these lens elements may be selected to provide the desired collimation.However, for the case shown in FIGS. 15 and 16 in which light of twodifferent wavelengths emerge from a single fiber-optic exit point 1782,the configuration in FIGS. 4A, 4B, and 5 cannot provide perfectcollimation if the lenses shown in the figures represent single lenselements. For example, if the lenses 141A and 142A are singlets (singlelens elements) and if the incoming light 220A is collimated, then thebeam expander 140A can provide perfectly collimated light 230A at onlyone wavelength. In FIG. 4B, if the lenses 141B and 142B in beam expander140B are singlet lenses and if the incoming light 220B is collimated,then perfect collimation of the output beam 230B can be provided at onlyone wavelength. In FIG. 5, if the beam launch 170 includes a singletpositive lens 176 and if the light is launched fiber ferrule 174, thenperfect collimation of the emitted light 252 is only possible for asingle wavelength of the light 250.

This limitation is particularly problematic when a fiber coupler 1750 inFIG. 16 is used to combine two different wavelengths. By combining twobeams of light in optical fiber, good alignment of the differentwavelengths of light is assured, but simple launch arrangements such asFIG. 5 cannot produce light collimated at both wavelengths if the lens176 is a singlet.

A way to get around this limitation is described in reference to FIG.20A, in which the fiber launch 170 of FIG. 5 is replaced by amulti-element achromatic lens assembly 2002. Achromatic lens are made bycombining two or more lens elements, each lens element made of adifferent glass, each of the different glasses having different index ofrefraction and a different dispersion, the dispersion being related to arate of change in the index of refraction with wavelength.

Another purpose for using two or more lens elements in the lens assembly2002 is to reduce aberrations. This is done by selecting particularglasses, curvatures, and thicknesses to minimize aberrations as well asto provide good collimation at two desired wavelengths.

A doublet of two lens elements 2014 in FIG. 20B or 2044 in FIG. 20C maybe used to make the beam expander achromatic. Other lens arrangements ofthree or more lens elements may be used in beam expanders 2020 and 2040to provide collimated light 2018 and 2050, respectively. Achromatic beamexpanders 2020 or 2040 may be used in combination with an achromaticfiber launch 2000 if desired. Achromatic beam expanders 2020 or 2040 mayalso provide collimation of multiple wavelengths using free-spacecombining optics such as beam splitters.

While the invention has been described with reference to exampleembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Moreover, the use of the terms first, second, etc. do not denoteany order or importance, but rather the terms first, second, etc. areused to distinguish one element from another. Furthermore, the use ofthe terms a, an, etc. do not denote a limitation of quantity, but ratherdenote the presence of at least one of the referenced item.

What is claimed is:
 1. A coordinate measurement device configured tosend a first beam of light to a target point, the target point having aposition in space, the target point returning a portion of the firstbeam as a second beam, the measurement device comprising: a first lightsource configured to emit a first light having a first wavelength; asecond light source configured to emit a second light having a secondwavelength, the second wavelength different than the first wavelength; afiber-optic coupler that includes at least a first port, a second port,and a third port, the first port configured to accept a first portion ofthe first light, the second port configured to accept a second portionof the second light, the third port configured to transmit a thirdlight, the third light including a portion of the first portion and aportion of the second portion; an optical system configured to transmita portion of the third light out of the coordinate measurement device asthe first beam, the optical system including an achromatic opticalelement configured to collimate the first beam at the first wavelengthand the second wavelength; a first motor and a second motor thattogether are configured to direct the first beam of light to a firstdirection, the first direction determined by a first angle of rotationabout a first axis and a second angle of rotation about a second axis,the first angle of rotation produced by the first motor and the secondangle of rotation produced by the second motor; a first angle measuringdevice configured to measure the first angle of rotation and a secondangle measuring device configured to measure the second angle ofrotation; a distance meter configured to measure a first distance fromthe coordinate measurement device to the target point based at least inpart on a third portion of the second beam received by a first opticaldetector; and a processor configured to provide three-dimensionalcoordinates of the target point, the three-dimensional coordinates basedat least in part on the first distance, the first angle of rotation, andthe second angle of rotation.
 2. The coordinate measurement device ofclaim 1, further comprising: a position detector assembly that includesa position detector, a fourth portion of the second beam passing ontothe position detector, the position detector configured to produce afirst signal in response to a position of the fourth portion on theposition detector; and a control system configured to send a secondsignal to the first motor and a third signal to the second motor, thesecond signal and the third signal based at least in part on the firstsignal, the control system configured to adjust the first direction ofthe first beam to the position in space of the target point.
 3. Thecoordinate measurement device of claim 1, wherein the distance meter isan absolute distance meter.
 4. The coordinate measurement device ofclaim 1, wherein the optical system is further configured to couple thethird portion of the second beam into the third port of the fiber-opticcoupler.
 5. The coordinate measurement device of claim 1, wherein thefiber-optic coupler further includes a fourth port.
 6. The coordinatemeasurement device of claim 5, wherein the fourth port is attached to alow-reflection termination.
 7. The coordinate measurement device ofclaim 1, wherein the first wavelength is between 780 nm and 850 nm. 8.The coordinate measurement device of claim 1, wherein the secondwavelength is a red wavelength or a green wavelength.
 9. The coordinatemeasurement device of claim 1, wherein the first wavelength is aninfrared wavelength and the second wavelength is a visible wavelength.10. The coordinate measurement device of claim 1, wherein the firstwavelength is a visible wavelength and the second wavelength is aninfrared wavelength.
 11. The coordinate measurement device of claim 1,wherein the first wavelength is a visible wavelength and the secondwavelength is a visible wavelength.
 12. A method for measuringthree-dimensional coordinates of a target point located at a position inspace, the method comprising steps of: providing a coordinatemeasurement device that includes a first light source that produces afirst light at a first wavelength, a second light source that produces asecond light at a second wavelength different than the first wavelength,a fiber-optic coupler that includes at least a first port, a secondport, and a third port, an optical system that includes an opticalelement achromatic at the first wavelength and the second wavelength, afirst motor, a second motor, a first angle measuring device, a secondangle measuring device, a distance meter, and a processor; coupling afirst portion of the first light into the first port; coupling a secondportion of the second light into the second port; transmitting a thirdlight from the third port, the third light containing a portion of thefirst portion and a portion of the second portion; transmitting aportion of the third light through the optical system out of thecoordinate measurement device as a first beam of light collimated at thefirst wavelength and the second wavelength; directing the first beam oflight in a first direction, the first direction determined by a firstangle of rotation about a first axis and a second angle of rotationabout a second axis, the first angle of rotation produced by the firstmotor and the second angle of rotation produced by the second motor;measuring the first angle of rotation with the first angle measuringdevice; measuring the second angle of rotation with the second anglemeasuring device; reflecting a portion of the first beam from the targetpoint as a second beam; measuring a first distance from the coordinatemeasurement device to the target point with the distance meter, themeasured distance based at least in part on a third portion of thesecond beam of light received by a first optical detector; determiningthree-dimensional coordinates of the target point based at least in parton the first distance, the first angle of rotation, and the second angleof rotation; and storing the determined three-dimensional coordinates.13. The method of claim 12, further comprising steps of: providing aposition detector assembly, the position detector assembly including aposition detector; sending a fourth portion of the second beam onto theposition detector; obtaining a first signal from the position detector,the first signal responsive to the position of the fourth portion on theposition detector; sending a second signal to the first motor andsending a third signal to the second motor, the second signal and thethird signal based at least in part on the first signal; and adjustingthe first direction of the first beam to the position in space of thetarget point.
 14. The method of claim 12, wherein the step of providinga coordinate measurement device further includes providing the distancemeter as an absolute distance meter.
 15. The method of claim 12, whereinthe step of measuring a first distance from the coordinate measurementdevice to the target point with the distance meter further includes astep of coupling the third portion of the second beam into the thirdport of the fiber-optic coupler.
 16. The method of claim 12, wherein thestep of providing a coordinate measurement device further includesproviding the fiber-optic coupler with a fourth port.
 17. The method ofclaim 16, wherein the step of providing a coordinate measurement devicefurther includes providing the fiber-optic coupler with the fourth portattached to a low-reflection termination.